Massive research efforts have recently been and currently being paid on optical elements such as frequency modulators and light wavelength conversion elements where periodic polarization-reversed regions (polarization-reversed structures) are formed in the inside of a ferroelectric. Stoichiometric lithium tantalate single crystals and stoichiometric lithium niobate single crystals are attracting attention as such ferroelectric single crystals.
In such a single crystal, there arises a problem that some formed neighboring polarization-reversed regions can be combined together and/or some formed polarization-reversed regions can become polarization-reversed once again (back-switch) depending on the conditions of forming the polarization-reversed regions.
In addition to the above-identified problem, a back-switch of polarization-reversed regions and/or an expansion of polarization-reversed regions have been observed during a working process of preparing modulators or wavelength conversion elements after forming polarization-reversed regions particularly when stoichiometric lithium tantalate single crystals are employed out of stoichiometric lithium tantalate single crystals and stoichiometric lithium niobate single crystals as listed above.
To cope with these problems, techniques relating to a method of inverting polarization by controlling the order of lattice points and those relating to a method of inverting polarization by controlling the defect density have been proposed (see, for example, Patent Document 1 and Patent Document 2).
The technique described in Patent Document 1 reduces polarization-reversed regions that are combined together and also the back-switch phenomenon by providing a control layer showing a low degree of order of lattice points on the face where an electric field is applied. On the other hand, the technique described in Patent Document 2 reduces polarization-reversed regions that are bonded together and also the back-switch phenomenon by providing a control layer showing a high defect density on the face where an electric field is applied.    Patent Document 1: JP 2005-148202-A    Patent Document 2: JP 2005-148203-A